Aspirin has inhibitory effects on platelet function and prostaglandin synthesis. Since alterations in platelet function and prostaglandin production could influence blood flow to ischemic myocardium, we tested the effects of aspirin on coronary collateral flow following acute coronary occlusion in dogs. Aspirin when administered prior to occlusion in the open-chest, anesthetized dog increased epicardial collateral flow at 4 hours after occlusion. The aspirin dose inhibited in vitro platelet aggregation. Thus aspirin given prior to coronary occlusion produced a small but statistically significant increase in epicardial collateral flow.